Hydraulic fracturing is a well stimulation technique designed to increase the productivity of a well by creating highly conductive fractures or channels in a producing formation surrounding the well. The process normally involves two basic steps: (1) injecting a fluid at sufficient rate and pressure to rupture the formation, thereby creating a crack (fracture) in the reservoir rock; and (2) thereafter placing a particulate material (propping agent) in the formation to maintain the fracture wall open by resisting forces tending to close the fracture. If stimulation is to occur, the propping agent must have sufficient mechanical strength to bear the closure stresses and provide relatively high permeability in the propped fracture.
With advances in drilling technology, it is currently possible to drill horizontal wellbores deep into hydrocarbon-producing reservoirs. Utilization of horizontal wellbores allows extended contact with a producing formation, thereby facilitating drainage and production of the reservoir. In order to enhance the production from a reservoir, it is often necessary to hydraulically fracture the reservoir through which the horizontal wellbore has penetrated.
Although horizontal wellbores allow more contact with the producing formation, some difficulties are encountered when horizontal wellbores are utilized which are not commonly experienced when vertical wells are used. Methods utilized in producing hydrocarbons from a formation or reservoir via vertical wells often prove to be inefficient when attempting to remove hydrocarbons from a reservoir where horizontal wellbores are being used. This inefficiency results in utilization of increased amounts of fluids used during enhanced oil recovery operations. This results in a dimunition in the amount of hydrocarbons removed from the formation or reservoir.
In order to obtain additional production from a formation penetrated by horizontal wellbores, it is often necessary to fracture different intervals of the formation and prop the fracture with a proppant. To this end, a suitable concentration of a particulate propping agent is generally entrained in the fracturing fluid. Rounded sands with uniform particle size distribution have been generally acknowledged to be a preferred propping agent. Glass spheres and metallic shot have also been widely used. Graham et al. in U.S. Pat. No. 3,399,727 disclosed a glass sphere proppant having voids therein which reduced the tendency of said spheres to settle in a fluid suspension utilized within a vertical wellbore. This patent is incorporated by reference herein.
The extent to which productivity or injectivity of a well is improved by fracturing depends on the propped width of the fracture and on the permeability of the propping material when fully loaded by natural compressive stresses. Thus, the distribution of a propping agent within the fracture must be sufficiently dense to bear the imposed load without crushing or embedding and yet not so dense as to seriously reduce permeability. Proppant distributions have been investigated ranging from a 5% partial monolayer to multilayer packs 5 to 6 times the diameter of a single particle.